Carbonates and Bicarbonates GLY 4200 Lab 8 - Fall, 2012

Carbonate Stability Carbonates are minerals consisting of metallic elements bonded to the carbonate group, CO32- Although anionic groups are usually stable, one exception to this is the reaction of a carbonate with acid:

Fizz The carbon dioxide is a gas and bubbles (or "fizzes") off This provides a good qualitative test for the presence of carbonate minerals in most samples Most carbonate minerals will fizz when a small (!) drop of acid is placed on the mineral However, it may be necessary to listen for the fizz and a few minerals react so slowly that the reaction is not detectable

Hydrous Carbonates Hydrous carbonates are often soluble in water Hydrous carbonates are frequently weathering products Some, like azurite and malachite, are indicator minerals for possible ore deposits Both are idiochromatic

Azurite Azurite = Deep blue

Malachite Malachite = green Secondary mineral in the oxidized zones of copper ore deposits This picture shows a botryoidal specimen

Botryoidal From the Greek botrys, meaning bunch or cluster of grapes

Banding in Malachite

Calcite Group Consists of divalent cations bonded to the carbonate anionic group Structure is a distorted NaCl type, compressed along a three-fold axis, and CN = VI All members of the calcite group belong to class bar3 2/m Cations and carbonate anionic groups form alternate layers

Dolomite Group The dolomite group is similar to calcite except that two distinct divalent cations are present in the structure These cations occupy separate layers For example in dolomite itself there is a layer of Mg2+ ions, a layer of carbonate ions, a layer of Ca2+ ions, a layer of carbonate ions, a layer of Mg2+ ions, a layer of carbonate ions, etc The ordering of the cations results in dolomite group minerals belonging to class bar3

Aragonite Group Minerals in this group have large divalent cations and each cation is coordinated to nine oxygens Calcium is borderline between the ratio required for CN = VI or IX so occurs in both the calcite and aragonite structures Aragonite is denser and is stable at higher pressures The aragonite group minerals are orthorhombic, 2/m 2/m 2/m

Bicarbonates Bicarbonates are minerals with the bicarbonate anion, HCO3- Trona is a typical saline lake deposit

Text Reference See chapter 17 in the text for more information on carbonates and bicarbonates, pp. 399-403 and 407-416